Even with the Sandbox theme you cannot simply copy and paste CSS from a different stylesheet, since the stylesheet refers to many graphical elements like background picture, header images etc. etc. Unless you download and install these elements on your blog and know HOW to reference them in CSS, simply copying and pasting will not do anything.

If what you were hoping for was to copy/paste a new theme, you could, I suppose, approach staff for a refund. They should make it absolutely clear in the signup process that this is NOT an option. All you can do is ask, though, because there are a lot of disclaimers around the CSS upgrade. You are essentially on your own to learn and to use it.